Heat exchanger tube, heat exchanger tube assembly, and methods of making the same

ABSTRACT

A tube assembly for use in a heat exchanger is made by arranging a first corrugated fin structure between one broad, flat side of a tube and a side sheet, and arranging a second corrugated fin between another broad and flat side of a tube and another side sheet. Compressive forces are applied to the opposite faces of the side sheets to place crests and troughs of the corrugated fin structures into contact with the side sheets and the broad, flat sides, and the assembly is brazed.

FIELD OF THE INVENTION

The present invention generally relates to tubes, and to fin and tubeassemblies for heat exchangers, and to methods for making the same.

BACKGROUND

Large scale heat exchangers incorporating discrete, individuallyreplaceable tube assemblies having a tube to convey a first fluid, andsecondary heat transfer surface area for a second fluid transferringheat to or from the first fluid, are well known. As an example, heatexchangers of this type functioning as heavy duty equipment radiators totransfer waste heat from engine coolant to air have been described inU.S. Pat. No. 3,391,732 to Murray, and U.S. Pat. No. 4,236,577 toNeudeck. The tube assemblies used in these heat exchangers have acentral finned section for heat exchange, and un-finned cylindrical endsections for insertion into sealing grommets.

Heat exchanger tube assemblies of the kind described above are typicallyconstructed of copper, with the extended air-side surfaces in the finnedregion being soldered to the tube. Copper provides the advantages ofhigh thermal conductivity, easy manufacturability, and good strength anddurability. However, the steadily increasing price of copper has led toa demand for alternate, lower cost materials.

Aluminum has replaced copper as the preferred material of constructionin other heat exchangers (automobile and commercial radiators, forexample), but has not successfully replaced copper in heavy duty heatexchangers of this kind Aluminum has substantially lower strength thancopper, leading to durability concerns. This is especially problematicin applications where individual tube assemblies need to be removed andinserted in the field, as damage is likely to occur during suchhandling. Furthermore, the bonding of aluminum components requiressubstantially higher temperatures than the soldering of copper, leadingto manufacturing difficulties. Thus, there is still room for improvement

SUMMARY

According to an embodiment of the invention, a tube assembly for a heatexchanger includes a tube having a flat section with spaced apart broadtube sides joined by opposing narrow tube sides. The tube assemblyfurther includes two fin structures, each having wave crests and troughsconnected by flanks, and two generally planar side sheets. Wave troughsof one fin structure are joined to one of the broad tube sides, and wavecrests of that fin structure are joined to a face of one of the sidesheets. Wave troughs of the other fin structure are joined to the otherbroad tube side, and wave crests of that fin structure are joined to aface of the other side sheet.

In some embodiments the tube includes cylindrical sections at thelengthwise ends of the tube, with the flat section arranged between thecylindrical sections. In some embodiments the tube, the fin structures,and the side sheets are joined by braze joints, and in some embodimentsthey are formed of one or more aluminum alloys. According to someembodiments the thickness of the broad tube sides is at least twice thethickness of the side sheets.

According to another embodiment of the invention, a tube assembly for aheat exchanger includes a fluid flow conduit extending in a lengthwisedirection over at least a portion of the tube assembly. The fluid flowconduit has a major dimension and minor dimension, both perpendicular tothe lengthwise direction, the minor dimension being substantiallysmaller than the major dimension. A continuous tube wall surrounds theflow conduit. Two generally planar side sheets are spaced equidistantlyfrom the continuous tube wall in the minor dimension direction, and areconnected to the tube wall by thin webs.

In some such embodiments the continuous tube wall defines a tube wallcentroidal moment of inertia with respect to an axis in the majordimension direction. In some embodiments the centroidal moment ofinertia of the tube assembly with respect to that axis is at least fivetimes the tube wall centroidal moment of inertia, and in someembodiments at least ten times.

In some embodiments a first cylindrical tube section is joined to thecontinuous tube wall at a first end of the flow conduit, and a secondcylindrical tube section is joined to the continuous tube wall at asecond end of the flow conduit. In some such embodiments the outerperimeter defined by the continuous tube wall is greater than the outerperimeter of at least one of the cylindrical tube sections.

According to another embodiment of the invention, a method of making aheat exchanger tube assembly includes providing a tube, first and secondcorrugated fin structures, and first and second generally planar sidesheets. The first corrugated fin structure is arranged between the firstside sheet and a first broad and flat side of the tube, and the secondcorrugated fin structure is arranged between the second side sheet and asecond broad and flat side of the tube. A compressive force is appliedto opposing sides of the side sheets to place crests and troughs of thefin structures into contact with the side sheets and the broad and flatsides, and braze joints are created between the first fin structure andthe first side sheet, the first fin structure and the first broad andflat side, the second fin structure and the second side sheet, and thesecond fin structure and the second broad and flat side.

In some such embodiments, the tube, fin structures, and side sheets areelevated in temperature in a vacuum environment to create the brazejoints. In other environments they are elevated in temperature in acontrolled inert gas environment. In some embodiments providing thetube, fin structures, and side sheets includes providing a materialcoated with a braze filler metal.

In some embodiments the compressive force is transmitted through a firstseparator sheet adjacent to the first side sheet, and through a secondseparator sheet adjacent to the second side sheet. In some suchembodiments the separator sheets have a coefficient of thermal expansionthat is generally matched to that of the tube, side sheets, and finstructures. In some embodiments the first separator sheet is one ofseveral separator sheets adjacent to the first side sheet.

According to another embodiment of the invention, a method of makingheat exchanger tube assemblies includes providing several tubes, severalcorrugated fin structures, and several generally planar side sheets.Each of the tubes is arranged between pairs of the corrugated finstructures, and each of the corrugated fin structures is arrangedbetween one of the tubes and one of the side sheets. The tubes,corrugated fin structures, and side sheets are arranged into a stack.Separator sheets are arranged between adjacent pairs of the side sheets,and adjacent to the side sheets at the outermost ends of the stack. Acompressive load is applied to the stack in the stacking direction.Braze joints are created at the points of contact between the corrugatedfin structures and the tubes, and between the corrugated fin structuresand the side sheets, and the brazed tube assemblies are removed from theseparator sheets.

In some such embodiments, the tubes, fin structures, and side sheets areelevated in temperature in a vacuum environment to create the brazejoints. In other environments they are elevated in temperature in acontrolled inert gas environment. In some embodiments providing thetubes, fin structures, and side sheets includes providing a materialcoated with a braze filler metal.

According to another embodiment of the invention, a tube for a heatexchanger includes a first cylindrical section extending from a firstend of the tube, a second cylindrical section extending from a secondend of the tube, and a flat section located between the ends and havingtwo broad and flat, spaced apart parallel sides joined by two relativelyshort sides. Transition regions are located between each of thecylindrical sections and the flat section. The intersections of thetransition regions and each of the broad and flat sides of the tubedefine curvilinear paths.

In some such embodiments the two relatively short sides are arcuate inprofile. In some embodiments each of the curvilinear paths includes anapex located at a center plane of the tube, and in some such embodimentsan arcuate path segment is located at the apex.

In some embodiments the transition region adjacent to one of thecylindrical sections extends over a length that is at least equal to thediameter of that section. In some embodiments the outer perimeter of theflat section of the tube is greater than the outer perimeter of at leastone of the cylindrical sections, and in some embodiments is at leasttwenty-five percent greater.

In some embodiments the flat tube section defines a tube major dimensionbetween outermost points of the two relatively short sides, and thecurvilinear paths are each longer than the tube major dimension. In someembodiments the tube is made from an aluminum alloy.

According to another embodiment of the invention, a heat exchanger tubeis formed from a round tube by reducing a diameter of the round tube ina first section of the round tube, and flattening a second sectionadjacent to the first section to define two spaced apart, broad and flatsides in the second section. In some embodiments the first sectionsterminates at an end of the tube. In some embodiments the second sectionis flattened after reducing the diameter of the first section.

In some embodiments the diameter of the first section is reduced by aswaging operation. In some embodiments the second section is flattenedby impacting that section in a stamping die. In some embodiments thetube is made from an aluminum alloy.

In some embodiments a mandrel is inserted into the tube prior toflattening the second section, and is removed from the tube afterflattening the second section.

In some embodiments, the diameter of a third section of the round tubeis reduced, the third section being adjacent to the second section. Insome such embodiments the third section terminates at a second end ofthe tube. In some embodiments the second section is flattened afterreducing the diameter of the third section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger tube assembly accordingto an embodiment of the invention.

FIG. 2 is an elevation view of the heat exchanger tube assembly of FIG.1.

FIG. 3 is a detail view of the portion of FIG. 2 bounded by the lineIII-III.

FIG. 4 is a plan view of the heat exchanger tube assembly of FIG. 1.

FIG. 5 is an exploded perspective view of the heat exchanger tubeassembly of FIG. 1.

FIG. 6 is an elevation view of a stack of heat exchanger tube assembliesbeing made according to an embodiment of the invention.

FIG. 7 is a plan view of certain components of the stack of FIG. 6.

FIG. 8 is a perspective view of a heat exchanger tube according to anembodiment of the invention.

FIG. 9 is a partial perspective view of a prior art heat exchanger tube.

FIG. 10 is a partial section view along the lines X-X of FIG. 8.

FIG. 11 is a section view along the lines XI-XI of FIG. 8.

FIG. 12 is a partial perspective view of the partially formed tube ofFIG. 8.

FIGS. 13A and B are diagrammatic views of a forming operation to producethe tube of FIG. 8.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

A heat exchanger tube assembly 1 according to an embodiment of theinvention is shown in FIGS. 1-5. Such a tube assembly 1 can be used asone of many individual tubes of a heat exchanger, for example aradiator, in large heavy duty equipment such as an excavator, miningtruck, gen-set, etc. It should be understood, however, that the tubeassembly 1 can be used in heat exchangers of various types and sizes.

The tube assembly 1 includes a tube 2 extending from a first end 7 to asecond end 8. The tube 2 defines a fluid flow conduit whereby a fluid(by way of example, engine coolant) can be transported through the tubeassembly 1. As one example, the tube assembly 1 can be used in an enginecoolant radiator in order to reject waste heat from a flow of enginecoolant as that flow of engine coolant flow through the tube 2 from oneof the ends 7, 8 to the other of the ends 7, 8.

The tube 2 includes a flat section 3 located between the ends 7, 8. Theflat portion 3 (best described with reference to FIG. 11) includes firstand second parallel, broad and flat sides 12. The broad and flat sides12 are spaced apart from one another, and are joined by two opposing,spaced apart, narrow tube sides 15. While the narrow tube sides 15 areshown as being arcuate in profile in the exemplary embodiment, in otherembodiments the narrow tube sides 15 can be straight, or they can be ofsome other profile shape. The two broad and flat sides 12 and the twonarrow sides 15 together define a continuous tube wall 25 of the fluidflow conduit, with an open spaces defined interior to the continuoustube wall 25 in order to allow for the flow of a fluid through the tube2. While none are shown in the exemplary embodiment, it can bepreferable in some cases to provide surface enhancement or flowturbulation features within the flow conduit in order to enhance therate of heat transfer between a fluid passing through the tube 2 and thetube wall 25.

Continuing with reference to FIG. 11, the flat section 3 of the tube 2has a tube minor dimension, d1, defined as the distance between theoutward-facing surfaces of the two broad and flat sides 12, and a tubemajor dimension, d2, defined as the distance between outermost points ofthe two narrow sides 15. In some highly preferable embodiments the majordimension, d2, is several times greater than the minor dimension, d1. Asan example, the major dimension of the exemplary embodiment is ninetimes greater than the minor dimension.

The tube assembly 1 further includes two convoluted fin structures 10arranged along the flat section 3. The fin structures 10 includemultiple flanks 16 connected in alternating fashion by crests 18 andtroughs 17 so that each of the fin structures 10 is of an approximatelysinusoidal shape (best seen in FIG. 3). The fin structures 10 can be ofa plain type, as shown in FIG. 3, or they can include additionalfeatures to increase heat transfer, structural strength, durability, orcombinations of the above. By way of example, in some embodiments thefin structures 10 can include louvers, bumps, slits, lances, or otherfeatures that are known to improve heat transfer and/or structuralrigidity of the flanks 16. In other embodiments, an edge hem can beprovided at one or both of the ends of a fin structure 10 adjacent thenarrow tube sides 15. Such an edge hem can be especially beneficial inproviding resistance to damage that may be caused by impingement ofrocks or other debris.

Thin side sheets 11 are also included in the tube assembly 1. These sidesheets 11 are parallel to the opposing broad and flat sides 12 of thetube 2, and are spaced equidistantly therefrom on either side by the finstructures 10. Accordingly, the flanks 16, crests 18, and troughs 17 ofthe fin structures 11 provide a plurality of thin webs to space the sidesheets 11 from the continuous tube wall 25. The side sheets 11 aregenerally planar, but can include features such as, for example, bentedges in order to provide increased stiffness and/or to aid in assembly.

The spaces between the flanks 16 provide flow channels for a fluid to beplaced in heat transfer relation with the fluid passing through the tube2, so that heat can be exchanged between the two fluids. As an example,ambient air can be directed through the flow channels in order to coolengine jacket coolant passing through the tube 2. It should beunderstood, however, that various other fluids can be placed in heattransfer relation using the tube assembly 1. Each of the flow channelsbetween the flanks 16 is further defined by one of the troughs 17 andcrests 18, and by one of the flat sides 12 of the tube 2 and thegenerally planar side sheets 11. By fully bounding the flow channels inthis manner, the fluid passing through those channels is prevented fromprematurely exiting the channels, thus improving the ability to transferheat.

The tube 2, fin structures 10, and side sheets 11 are preferably bondedtogether to form a monolithic structure in order to provide both goodthermal contact between the fluids to be placed in heat transferrelation, and good structural integrity. While a variety of materialscan be used to construct the tube assembly 1, in highly preferableembodiments the tube 2, fin structures 10, and side sheets 11 are formedfrom metals having a high thermal conductivity, such as aluminum,copper, and the like. The components can be bonded together to form thetube assembly 1 by a variety of processes including brazing, soldering,gluing, etc.

In order to promote good heat transfer between the fluids, it can beadvantageous for the fin structures 10 and the side sheets 11 to extendover the full major dimension d2 of the flat section 3. In some cases,it may be preferable to extend the fin structures 10 and the side sheets11 slightly beyond the outer edges of the arrow tube sides 15 in orderto protect the fluid flow conduit from damage by impingement of rocks orother debris.

The inclusion of even very thin side sheets 11 has been found to greatlystiffen the tube assembly 1, especially with respect to bending aboutthe centroidal axis in the tube major dimension d2. The fin structures10 provide very little stiffness in this direction due to theirconvoluted nature, so that, in the absence of the side sheets 11, thecontinuous tube wall 25 provides the only resistance to bending aboutthat centroidal axis. Due to the relatively small minor dimension d1 ofthe flat tube section 3, the resistance to bending about that centroidalaxis by the continuous tube wall 25 alone is fairly small, and thespacing of the side sheets 11 away from that centroidal axis by adistance substantially greater than the minor dimension d1 providessubstantial benefit.

The impact of the side sheets 11 on the bending stiffness of the tubeassembly 1 about the centroidal axis in the tube major dimension d2 canbe quantified by comparing the centroidal moment of inertia about thataxis of the tube assembly 1 to that of the tube 2 alone (the finstructures 10 can be assumed to provide no contribution to thecentroidal moment of inertia, other than by maintaining the offset ofthe side sheets 11 from the flat sides 12 of the tube 2). For anexemplary embodiment having a tube wall thickness of 0.8 mm, a sidesheet thickness of 0.25 mm, a fin structure height of 6.55 mm, a minordimension of 3.7 mm, and a major dimension of 23.27 mm, the centroidalmoment of inertia about the tube major dimension axis for the tubeassembly and the tube alone are calculated to be 925 mm⁴ and 76 mm⁴,respectively. In other words, the centroidal moment of inertia of thetube assembly about the tube major dimension axis is approximatelytwelve times that of the tube itself. In preferable embodiments thecentroidal moment of inertia of the tube assembly about the tube majordimension axis is at least five times that of the tube itself, and inhighly preferable embodiments, at least ten times. This is especiallypreferable when the tube 2 is constructed of a material exhibitingrelatively low modulus of elasticity, for example, alloys of aluminum.

The tube 2 of the exemplary embodiment further includes a firstcylindrical section 4 adjacent to the first end 7, and a secondcylindrical section 5 adjacent to the second end 8, with the flatsection 3 arranged between the first and second cylindrical sections.These cylindrical sections 4, 5 allow for reliable and leak-freeinsertion of the tube assembly 1 into receiving grommets arranged inopposing headers of a heat exchanger (not shown). In order to maximizethe amount of the tube available for effective heat transfer, the lengthof the cylindrical end sections are preferably kept to a minimum, andthe length of the flat section 3 is preferably 90% or more of theoverall length of the tube 2. A circumferential bead 9 is provided inthe cylindrical section 5 of the exemplary embodiment in order to limitthe downward movement of the tube assembly 1 when vertically arranged ina heat exchanger.

While the embodiments shown in the accompanying figures include thecylindrical end sections at both ends of the tube, it should beunderstood that in some instances a tube assembly 1 can be devoid of oneor both cylindrical end sections 4, 5. When such cylindrical endsections are not included, the corresponding receiving grommets can beprovided with receiving openings that correspond to the profile of thecontinuous tube wall 25 in the flat section 3.

In certain preferable embodiments of the invention, a heat exchangertube assembly 1 is made by creating braze joints between an aluminumtube 2, first and second aluminum corrugated fin structures 10, andfirst and second aluminum side sheets 11. The first corrugated finstructure 10 is arranged between the first side sheet 11 and a firstbroad and flat side 12 of the tube 2, while the second corrugated finstructure 10 is arranged between the second side sheet 11 and a secondbroad and flat side 12 of the tube 2. The assembly is compressed inorder to place crests 18 and troughs 17 of the fin structures 10 incontact with the adjacent parts so that braze joints can be formed atthe points of contact.

A brazing filler metal having a melting temperature that is lower thanthe melting temperatures of the tube 2, fin structures 10, and sidesheets 11 is used to create the braze joints. Such a filler metal istypically aluminum with small quantities of other elements (silicon,copper, magnesium, and zinc, for example) added to reduce the meltingtemperature. The braze filler metal can advantageously be provided as acoating on one of more of the components to be brazed. In someembodiments, both sides of the sheet material used to form thecorrugated fin structures 10 is coated with the braze filler metal,thereby providing the required braze filler metal at all of the contactpoints where braze joints are desired while avoiding having braze fillermetal at locations where joints are not necessary or undesirable.

While many methods can be used to elevate the temperature of the tube 2,the fin structures 10, and the side sheets 11 in order to melt the brazefiller metal and form the braze joints, two especially preferablemethods are vacuum brazing and controlled atmosphere brazing. In vacuumbrazing, the assembled parts are placed into a sealed furnace andsubstantially all of the air is removed in order to create a vacuumenvironment. In this process, magnesium present in the alloys isreleased as the parts are heated and serves to break up the oxide layerpresent on the external surfaces of the components, allowing the moltenbraze filler metal to bond to the exposed aluminum. The oxide layer isprevented from reforming and interfering with the metallurgical bondingby the absence of oxygen in the vacuum environment.

In controlled atmosphere brazing, flux is applied to the componentsprior to heating. Heating of the parts occurs in an inert gasenvironment in order to prevent the re-formation of the oxide layerafter the flux reacts and displaces the oxide layer present on themating surfaces of the parts. With the oxide layer displaced, the moltenbraze filler metal bonds to the exposed aluminum in order to create thebraze joints.

It can be especially preferable to braze several of the tube assemblies1 at one time in order to increase throughput in a productionmanufacturing environment. FIG. 6 illustrates a method according to anembodiment of the invention wherein four tube assemblies 1 are madesimultaneously. It should be understood that the same method can be usedto make more than four or fewer than four of the tube assemblies at atime.

In the embodiment of FIG. 6, tubes 2, corrugated fin structures 10, andgenerally planar side sheets 11 are provided. Each of the tubes 2 isarranged between pairs of the corrugated fin structures 10, and each ofthe corrugated fin structures 10 is arranged between one of the tubes 2and one of the generally planar side sheets 11. Separator sheets 19 arearranged between adjacent pairs of the generally planar side sheets 11.The tubes 2, corrugated fin structures 10, and generally planar sidesheets 11 are arranged into a stack 26. Additional separator sheets 19are arranged adjacent to the generally planar side sheets 11 at theoutermost ends of the stack 26, and a compressive load is applied to thestack 26 in the stacking direction in order to place the crests 18 andthe troughs 17 of the convoluted fin structures into contact with theadjacent side sheets 11 and broad and flat sides 12 of the tubes 2.

In order to provide a uniform compressive load to the stack 26, bars 21having a high stiffness (for example, structural steel channels) can beused on the outermost ends of the stack 26. The compressive load can bemaintained after it has been applied to the stack through the use ofmetal bands 22 that surround the stack 26 in several locations. Thebands 22 are tightened over the bars 21 while the stack 26 iscompressed, so that tension in the bands 22 maintains the compressiveload. After having been so assembled, the stack 26 is placed into abrazing furnace in order to create the individual tube assemblies 1. Thestack 26 is heated within the furnace to a temperature suitable formelting the braze filler metal, after which the stack 26 is cooled inorder to re-solidify the melted braze filler metal, thereby creatingbraze joints at the contact points. After cooling, the individual tubeassemblies 1, having been brazed into individual monolithic structures,can be removed from the separator sheets 19. The separator sheets 19 canbe provided with a coating to prevent any metallurgical bonding betweenthe separator sheets 19 and the side sheets 11, as such undesirablebonding can otherwise occur at brazing temperature even without thepresence of braze filler metal.

As the stack 26 is heated to a brazing temperature, thermal expansion ofthe metal materials in the stack 26 will occur. In aluminum brazing, thecomponents are typically heated to a brazing temperature of 550° C. to650° C. This temperature range is substantially higher than that used tosolder copper components, and consequently the thermal expansionexperienced by the components of the tube assemblies 1 during thebonding process is substantially greater if the components are aluminumthan if they are copper.

The inventors have found that care must be taken during the brazingprocess to ensure that the fin structures 10 are not distorted by theheating to brazing temperature and cooling back down to ambienttemperature. Unlike in traditional brazed aluminum radiatormanufacturing, involving multiple rows of tubes and fin structuresjoined together into a monolithic brazed core, the flanks 16 of the finstructures 10 are prone to distortion by shearing forces introducedthrough thermal expansion differences between the components of the tubeassemblies 1 and the separator sheets 19. In some embodiments of theinvention, this problem is remedied by generally matching the thermalexpansion coefficient of the separator sheets 19 match that of the tubes2, fin structures 10, and side sheets 11. This can be achieved byforming the separator sheets 19 from similar aluminum alloys, or fromanother material exhibiting a similar rate of thermal expansion.

Alternatively, or in addition, multiple individual separator sheets 19can be used between each adjacent tube assembly 1, as shown in FIG. 7.Gaps 20 are provided between adjacent ones of the individual separatorsheets 19. In the case where the separator sheets 19 are constructed ofa material having a substantially different coefficient of thermalexpansion than the materials from which the tubes 2, fin structures 10,and side sheets 11 are constructed, the gaps 20 can increase or decreaseduring the heating and cooling of the stack 26, thereby substantiallyalleviating the distortion of the fin structures 10 that might otherwiseresult from the mismatch in thermal expansion coefficients. The gaps 20serve as breaks to avoid the accumulation of the thermal expansioninduced distortion, so that any such distortion is limited to thediscrete contact areas underneath each of the individual separatorsheets 19. The assembly method depicted in FIG. 7 can be especiallybeneficial when a more temperature resistant material such as stainlesssteel is used for the separator sheets 19, and the components of thetube assemblies 1 are made from aluminum.

The tube 2 will now be discussed in greater detail, with specificreference to FIGS. 8-13. As described previously, the embodiment of thetube 2 shown in FIG. 8 includes a flat tube section 3 located between afirst cylindrical tube section 4 and a second cylindrical tube section5. The first cylindrical tube section 4 extends from the first end 7 ofthe tube 2, while the second cylindrical tube section 5 extends from thesecond end 8 of the tube 2. Transition regions 6 are located between theflat section 3 and each of the cylindrical sections 4 and 5. Thetransition regions 6 provide a smooth continuous flow path for a fluidpassing through the tube 2, as well as avoiding locations of mechanicalstress concentration in the tube material.

As shown in detail in the partial sectional view of FIG. 10, atransition region 6 extends over a length L, spanning from a location 27proximal to the end 7 of the tube 2 to a location 14 distal to the end7. The length L is preferably at least equal to the diameter of thecylindrical end section 4, although in some alternative embodiments itmay be smaller in size than the diameter of the corresponding endsection. As seen in FIG. 8, the broad and flat side 12 extends past thelocations 14 at either end so that at least a portion of the broad andflat side 12 is located along the tube 2 between the locations 27 and 14that define the beginning and end of a transition region 6.

In preferable embodiments, the intersections of the transition regions 6and the broad and flat sides 12 of the flat tube region 3 definecurvilinear paths 13. These curvilinear paths 13 provide a beneficialstiffening of the flat section 3 of the tube 2 with respect to a bendingmoment about the tube major dimension axis. For purposes of comparison,a prior art tube 102 is shown in FIG. 9 and includes a flat section 103joined to a cylindrical section 104 by way of a transition section 106.The intersection of the transition region 106 and the flat section 103defines a straight path 113 on the broad and flat side 112 of the flatsection 103. The straight path 113 extends in the tube major dimension,and bending about the major dimension axis is fairly easy. This can beespecially detrimental during the installation and/or removal of a tubeassembly containing the tube 102 from a heat exchanger, as suchinstallation and such removal frequently applies bending moments of thistype onto the tube. This problem is especially exacerbated when the tubeis constructed of a fairly low strength material such as annealedaluminum.

The inventors have found that the curvilinear path 13 provides asubstantial stiffening effect to resist a bending moment of theaforementioned type, and prevents buckling or other damage to the tube 2during installation, removal, and other handling of the tube 2 or a tubeassembly 1 containing a tube 2. While benefit can be derived from anynon-linear path, it can be especially beneficial for the path 13 to bedefined by a series of connected arcuate path segments.

In the exemplary embodiment, the curvilinear paths 13 each include anapex located at the approximate center plane of the tube, so that theapex is located at the point 14 along the path 13 that is furthermostaway from the end 7 (in the case of the transition region between theflat section 3 and the first cylindrical end 4) or the end 8 (in thecase of the transition region between the flat section 3 and the secondcylindrical end 5). The path 13 preferably includes an arcuate pathsegment at the apex so that stress concentrations are avoided at theapex.

In some preferable embodiments, the outer perimeter (i.e. circumference)of at least one of the two cylindrical sections 4, 5 is less than theouter perimeter of the continuous tube wall 25 in the flat section 3.This advantageously allows for a relatively large heat transfer surfacearea per unit length in the flat section 3, without requiring acorrespondingly large diameter at one or both of the ends 7, 8. Asmaller diameter at the ends can be preferable, as it can enable closerspacing of adjacent tube assemblies and requires less sealing surface atthe ends, for example. In some preferable embodiments the outerperimeter of the flat section 3 exceeds the outer perimeter of at leastone of the two cylindrical end sections by at least 25%.

Heat exchangers including fluid conveying tubes having a flattenedprofile over the entirety of their length are well-known in the art,having been used for decades as radiators and the like. Flat tubes ofthis type are usually constructed in one of two ways. They are eitherextruded and/or drawn in the flat shape from a billet of material andcut into discrete lengths, or they are created in a tube mill fromcoiled sheet by forming the sheet form into a round shape, seam welding,roll flattening to the flat tube shape, and cutting into discrete tubelengths.

In the case of tubes such as the prior art tube 102 (FIG. 9) having aflattened section 103 and a cylindrical end section 104, the ends of theflat tube are formed into a cylindrical shape to form the cylindricalend section 104, and the transition section 106. This operation can beperformed quickly and easily when the tube is constructed from highlymalleable material such as copper, and only requires the extreme ends ofthe tube 2 to be formed. However, this method is not capable ofachieving a transition section 6 as previously described.

The transition regions 6 can be formed by initially forming the tube 2in a round form having an outer diameter equal to the desired outerperimeter of continuous tube wall 25 in the flat section 3. Next, withspecific reference to FIG. 12, the ends of the round tube 2 are reducedin diameter to form the cylindrical ends 4 and 5, as well as a taperedtransition region 6′ between the ends 4, 5 and the central section 3′which retains the original round shape. This reduction in diameter canbe accomplished by, for example, swaging of the tube ends. In somepreferable embodiments the ends are reduced in diameter by at least 20%in order to achieve the desired ratio of outer perimeters between theflat section 3 and the cylindrical end sections 4, 5.

As depicted in FIGS. 13A and 13B, the profile of the flat section 3 ofthe tube 2 can be defined by forming that portion 3′ of the tube 2between a first forming die half 22 and a second forming die half 23.The tube 2 is inserted between the die halves 22, 23 when the die is inan open position, i.e. when the two die halves are separated from oneanother, as in FIG. 13A. With the tube 2 so located, the die closes soas to be in the closed position of FIG. 13B, thereby forming the flatsection 3 of the tube 2 to the minor dimension d1 and the majordimension d2. Optionally, a mandrel 24 can be placed within the tube 2prior to the forming operation in order to prevent buckling or otherundesirable deformation of the broad and flat tube walls 12 during theforming operation. The mandrel 24, when used, can be removed from thetube 2 after the forming operation is complete. The geometry of thetransition regions 6 can be produced by including complementary negativerepresentations of the geometry in the contacting faces of the diehalves 22 and 23, so that the desired geometry of the transition regions6 is formed into the tube 2 during the forming operation.

Various alternatives to the certain features and elements of the presentinvention are described with reference to specific embodiments of thepresent invention. With the exception of features, elements, and mannersof operation that are mutually exclusive of or are inconsistent witheach embodiment described above, it should be noted that the alternativefeatures, elements, and manners of operation described with reference toone particular embodiment are applicable to the other embodiments.

The embodiments described above and illustrated in the figures arepresented by way of example only and are not intended as a limitationupon the concepts and principles of the present invention. As such, itwill be appreciated by one having ordinary skill in the art that variouschanges in the elements and their configuration and arrangement arepossible without departing from the spirit and scope of the presentinvention.

We claim:
 1. A method of making a heat exchanger tube assembly,comprising: providing a tube having first and second opposing broad andflat sides; providing first and second corrugated fin structures;providing first and second side sheets; arranging the first corrugatedfin structure between the first side sheet and the first broad and flatside of the tube; arranging the second corrugated fin structure betweenthe second side sheet and the second broad and flat side of the tube;applying a compressive force to opposing sides of the first and secondside sheets to place crests and troughs of the first corrugated finstructure into contact with the first side sheet and the first broad andflat side of the tube, and to place crests and troughs of the secondcorrugated fin structure into contact with the second side sheet and thesecond broad and flat side of the tube; and creating braze jointsbetween the first corrugated fin structure and the first side sheet, thefirst corrugated fin structure and the first broad and flat side of thetube, the second corrugated fin structure and the second side sheet, andthe second corrugated fin structure and the second broad and flat sideof the tube; wherein applying the compressive force includestransmitting the compressive force through a first plurality ofseparator sheets arranged adjacent the first side sheet, and through asecond plurality of separator sheets arranged adjacent the second sidesheet, such that the first side sheet is located between the firstplurality of separator sheets and the first corrugated fin structure,and the second side sheet is located between the second plurality ofseparator sheets and the second corrugated fin structure.
 2. The methodof claim 1, wherein creating braze joints includes elevating thetemperature of the tube, the first and second corrugated fin structures,and the first and second side sheets in a vacuum environment.
 3. Themethod of claim 1, wherein creating braze joints includes elevating thetemperature of the tube, the first and second corrugated fin structures,and the first and second side sheets in a controlled inert gasatmosphere environment.
 4. The method of claim 1, wherein at least oneof providing a tube, providing a corrugated fin structure, and providinga side sheet includes providing a material coated with a braze fillermetal.
 5. The method of claim 1, wherein the first plurality ofseparator sheets and the second plurality of separator sheets have acoefficient of thermal expansion that is generally matched to thecoefficients of thermal expansion of the tube, the first and secondcorrugated fin structures, and the first and second side sheets.
 6. Themethod of claim 1, wherein the first plurality of separator sheets andthe second plurality of separator sheets have a coating to prevent themetallurgical bonding between said separator sheets and the side sheets.7. A method of making heat exchanger tube assemblies, comprising:providing a plurality of tubes having first and second opposing broadand flat sides; providing a plurality of corrugated fin structures;providing a plurality of side sheets; arranging each of the tubesbetween pairs of the corrugated fin structures; arranging each of thecorrugated fin structures between one of the tubes and one of the sidesheets; assembling the tubes, corrugated fin structures, and side sheetsinto a stack; arranging separator sheets between adjacent pairs of theside sheets; arranging separator sheets adjacent the side sheets at theoutermost ends of the stack; applying a compressive load to the stack inthe stacking direction so as to place crests and troughs of thecorrugated fin structures into contact with adjacent ones of the sidesheets and with adjacent ones of the broad and flat sides of the tubes;creating braze joints at points of contact between the corrugated finstructures and the side sheets, and at points of contact between thecorrugated fin structures and the tubes; and removing the brazed heatexchanger tube assemblies from the separator sheets.
 8. The method ofclaim 7, wherein creating braze joints includes elevating thetemperature of the stack in a vacuum environment.
 9. The method of claim7, wherein creating braze joints includes elevating the temperature ofthe stack in a controlled inert gas atmosphere environment.
 10. Themethod of claim 7, wherein at least one of providing a plurality oftubes, providing a plurality of corrugated fin structures, and providinga plurality of side sheets includes providing a material coated with abraze filler metal.
 11. The method of claim 7, wherein the separatorsheets have a coefficient of thermal expansion that is generally matchedto the coefficients of thermal expansion of the tubes the corrugated finstructures, and the side sheets.
 12. The method of claim 9, wherein theseparator sheets have a coating to prevent the metallurgical bondingbetween said separator sheets and the side sheets.
 13. The method ofclaim 1, wherein a first bar is placed adjacent the first plurality ofseparator sheets and a second bar is placed adjacent the secondplurality of separator sheets and wherein the applying the compressiveforce includes transmitting the compressive force through the first barand through the second bar.
 14. The method of claim 13, furtherincluding maintaining the compressive force after applying thecompressive force by tightening at least one band across the first bar,the tube, and the second bar.
 15. The method of claim 1, wherein atleast a first gap is located within the first plurality of separatorsheets and at least a second gap is located within the second pluralityof separator sheets.
 16. A method of making a heat exchanger tubeassembly, comprising: providing a tube having first and second opposingbroad and flat sides; providing first and second corrugated finstructures; providing first and second side sheets; arranging the firstcorrugated fin structure between the first side sheet and the firstbroad and flat side of the tube; arranging the second corrugated finstructure between the second side sheet and the second broad and flatside of the tube; applying a compressive force to opposing sides of thefirst and second side sheets to place crests and troughs of the firstcorrugated fin structure into contact with the first side sheet and thefirst broad and flat side of the tube, and to place crests and troughsof the second corrugated fin structure into contact with the second sidesheet and the second broad and flat side of the tube; and creating brazejoints between the first corrugated fin structure and the first sidesheet, the first corrugated fin structure and the first broad and flatside of the tube, the second corrugated fin structure and the secondside sheet, and the second corrugated fin structure and the second broadand flat side of the tube, wherein applying the compressive forceincludes transmitting the compressive force through a first separatorsheet arranged adjacent the first side sheet, and through a secondseparator sheet arranged adjacent the second side sheet, such that thefirst side sheet is located between the first separator sheet and thefirst corrugated fin structure, and the second side sheet is locatedbetween the second separator sheet and the second corrugated finstructure, and wherein the first and second separator sheets have acoefficient of thermal expansion that is generally matched to thecoefficients of thermal expansion of the tube, the first and secondcorrugated fin structures, and the first and second side sheets.
 17. Themethod of claim 16, wherein a first bar is placed adjacent the firstseparator sheet and a second bar is placed adjacent the second separatorsheet, and wherein the applying the compressive force includestransmitting the compressive force through the first bar and through thesecond bar.
 18. The method of claim 16, wherein the first and secondseparator sheets have a coating to prevent the metallurgical bondingbetween the separator sheets and the side sheets.
 19. The method ofclaim 16, further including maintaining the compressive force afterapplying the compressive force by tightening at least one band acrossthe first bar, the tube, and the second bar.
 20. The method of claim 16,further including arranging a third separator sheet adjacent to thefirst separator sheet and arranging a forth separator sheet adjacent tothe second separator sheet, wherein a first gap is located between thefirst separator sheet and the third separator sheet and a second gap islocated between the second separator sheet and the forth separatorsheet.